1. Field of the Invention
The present invention relates to a valve timing control device for an internal combustion engine, the device comprising a variable valve timing mechanism for controlling the valve overlap period.
2. Description of the Related Art
When the valve overlap period, during which the intake valve and the exhaust valve are opened simultaneously at the end of an exhaust stroke, is made long, the trapping efficiency and the scavenging efficiency increase so that a good engine performance can be obtained. On the other hand, in engine operating conditions under which the degree of opening of the throttle valve is relatively small so that the negative pressure in an intake port becomes high, once the valve overlap period is made long, the amount of back-flow exhaust gas into the intake port becomes large so that combustion deteriorates. Accordingly, it is desirable that the valve overlap period is varied in accordance with the engine operating condition. For this purpose, a variable valve timing mechanism capable of varying the valve overlap period has already been suggested.
According to the usual valve timing control device for controlling the valve overlap period by means of the variable valve timing mechanism, in engine operating conditions under which the degree of opening the throttle valve is relatively small, the valve overlap period is made short to prevent deterioration of combustion caused by a large amount of back-flow exhaust gas. However, Japanese Unexamined Patent Publication Nos. 59-103910 and 2-298614 disclose a valve timing control device which makes the valve overlap period long in an idle condition when the engine has not warmed up. This is intended to positively utilize the back-flow of exhaust gas, as exhaust gas recirculation, by leading a part of the exhaust gas into the intake port to reduce the amount of NOx in the exhaust gas, by lowering the combustion temperature, and to raise the intake air temperature by heating the intake port.
However, in the idle condition when the engine has not warmed up, combustion is very unstable so that the engine can stall when the amount of back-flow exhaust gas is increased. The valve timing control device disclosed in the above No. 2-298614 increases the amount of fuel and delays the ignition time to improve the combustion at this time. However, these cannot improve the combustion sufficiently so that an engine stall can still occur.